ok, i've had an even better idea for a potentially more elegant solution.
TT is (wonderfully) only one board. the i2c "bus board" could be a thin mezzanine, that connects to both the power jack (with a 5x2 female connector) and the i2c jack (with a 3x2 female connector).
there would then be a load of 3x2 male connectors on the back for i2c, and a male power connector (so there won't need to be redundant cabling.) the extra regulator and resistors can be hidden on the board. all in all it should be an elegant (and cheap) solution.
of course, i've already ordered some of yesterday's version and that will also be totally fine. so i'll only make time to draw up this new version if there are other people interested.